There is known an oil pump of an internal contact gear pump type having a driving gear rotated about its axis by an engine, an annular driven gear having internal teeth meshing with external teeth of the driving gear and rotatable by the driving gear about an eccentric axis eccentric with respect to the axis of the driving gear, a pump chamber accommodating the driving gear and the driven gear, and a housing having a plurality of pressure chambers which are open in a side surface of the pump chamber to discharge a working oil from the pump chamber and which are formed between mutually meshing surfaces of the external and internal teeth such that the pressure chambers are spaced apart from each other in a circumferential direction of the driving and driven gears, the housing further having a high-pressure discharge passage and a low-pressure discharge passage which communicate with the plurality of pressure chambers one after another in the process of decrease of volume of each of the pressure chambers while the pressure chambers are moved in a direction of rotation of the driving and driven gears, the oil pump being operated such that a pressure of the working oil to be discharged through the low-pressure discharge passage is held lower than a pressure of the working oil discharged from the high-pressure discharge passage by a predetermined amount, when an amount of consumption of the working oil of a relatively high pressure can be afforded by only an amount of discharge of the working oil from the high-pressure discharge passage. Patent Documents 1 and 2 disclose examples of such type of oil pump. This type of oil pump wherein the pressure of the working oil to be discharged from the low-pressure discharge passage is held at the above-indicated low level requires a relatively low driving torque, and accordingly improves fuel economy of a vehicle equipped with the oil pump.
The oil pump disclosed in the Patent Document 2 has an oil relief groove formed in the side surface of the pump chamber, for communication of the low-pressure discharge passage with a specific pressure chamber of the pressure chambers which is located between the openings of the high-pressure and low-pressure discharge passages and is not in communication with the discharge passages in the absence of the oil relief groove. The oil relief groove permits the working oil to flow from the specific pressure chamber into the low-pressure discharge passage, preventing an abrupt increase of the pressure of the working oil in the specific pressure chamber even when the specific pressure chamber is moved past the above-indicated location between the high-pressure and low-pressure discharge passages. Thus, the oil relief groove prevents an increase of the required pump drive torque due to the increase of the pressure of the working oil in the specific pressure chamber.